Please first check the data storage directories of your MongoDB. While doing some crazy tests and more or less intentionally made the MongoDB go crash I found at times that it "forgot" about the available data, even though there were still kind of big files probably with all the data. I don't know how to recover from there but I assume there would be some articles out there in the internet helping you with it.
But yeah, mongodump, borg backup, lftp synchronize to remote drive (or similar). Every day. And if you're at it, make sure the backups on your local computer are working as intended as well